This invention relates to a method for the direct metal plating of nonconductive substrates and, in particular, to the regeneration of solutions used in the plating process.
The metal plating of plastics and other nonconductive (dielectric) substrates is well-known in the art and the following description will be directed for convenience to plastics, especially epoxy, and the preparation of printed circuit boards.
The preparation of printed circuit boards requires the plating of conductive metal layers, usually copper, onto the plastic substrate of the board. These boards vary in design and may have a copper layer on each surface of the epoxy (two-sided boards) or they can be multi-layer boards which have a plurality of inter-leaved parallel planar copper and epoxy layers. In both types, through-holes are drilled in the board and metal plated to facilitate connection between the circuits on the copper layers.
In general and for purposes of illustration, the electroless metal plating method for the manufacture of through-hole printed circuit boards is a sequence of steps commencing with a commercially available copper clad laminate or multi-laminate made from, e.g., paper-epoxy, glass-epoxy di, multi or tetrafunctional epoxy, or glass polyimide material. A predesigned series of through-holes is formed in the board in any conventional manner (e.g. drilled and deburred), then preferably treated to clean and condition the plastic for adhesion and activation of the metal plating using a swell and etch technique. The board is now ready for electroless metal plating using conventional procedures.
A preferred step before plating, if not done previously, is to dissolve some of the glass fibers in the epoxy using, for example, acid fluorides, and then to pretreat the board by cleaning and conditioning with a composition such as ENPLATE.RTM.PC-475 to remove hydrocarbon soils and oxides and to enhance the surface for activation. The surface may also be microetched to enhance copper to copper adhesion. Thereafter the board is immersed in a catalyst, such as a tin-palladium solution, which activates the surface of the epoxy for electroless copper plating. ENPLATE Activator 444 sold by Enthone-OMI, Inc., West Haven Conn., is exemplary of this type catalyst. Following a water rinse, the laminate is immersed in an accelerator such as ENPLATE PA-491 to activate the catalyst by exposing the metal palladium ions on the board. After a water rinse, the board is dried and is immersed in an electroless copper plating solution for a period of time sufficient to plate copper to the desired thickness on the board surfaces and the through-hole connections. ENPLATE CU-700 and other similar plating compositions may be employed. The boards may then be electroplated using conventional techniques to provide a thicker final coating.
The above procedure, however, is time consuming and costly and it is preferred to improve the process by eliminating the need for electroless copper plating of the board prior to electrolytic plating. This process is generally known as direct metallization and in a commercial direct metallization process, after the drilling, deburring and cleaning steps, the board is conditioned using an adhesion promoter such as an alkaline permanganate solution, followed by treating with a cleaner conditioner (such as ENVISION DPS-5625) to remove oxides and oils and promote absorption of colloid. Activation is a two-step procedure consisting of a pre-dip to prevent contamination of the activator by drag-in (such as ENVISION DPS Carrier 5630) followed by contacting the board with a proprietary noble metal containing catalyst which is adsorbed onto the board. The catalyzed board is now specially treated with a proprietary post activator (also known as accelerators or generators) to leave a highly conductive and catalytic surface and a final acid wash stabilizes the conductive film. The board may now be directly metal plated by electrolytic plating. The board may also be electrolessly plated, e.g., flash coated, although for most applications this step is not performed and the board directly electrolytically plated.
Unfortunately, when the catalyzed board (palladium and tin are usually the catalysts) is treated with the post activator some of the tin is dissolved and builds up in the post-activator solution. The use of tin "contaminated" solutions has been found by us to adversely affect the subsequent electrolytic plating and the tin level is generally controlled by diluting with fresh post-activator solution (feed and bleed) or by bail out of the old solution and replenishment with fresh solution. These methods are costly and environmentally unsound and new methods are needed.
It is an object of the invention to provide an improved direct metallization process by regenerating for reuse the post-activator solutions used in the direct metallization process.
It is a further object of the invention to provide a method for regenerating for reuse the post-activator solution used in direct metallization processes.
Other objects and advantages will be apparent for the following description.